Re: [PATCH RFC] xen/swiotlb: avoid arch_sync_dma_* on per-device DMA memory

[Peng Fan <peng.fan@xxxxxxxxxxx>]

Hi Juergen,

On Mon, Apr 20, 2026 at 02:19:34PM +0200, Jürgen Groß wrote:
>On 15.04.26 17:08, Peng Fan (OSS) wrote:
>> From: Peng Fan <peng.fan@xxxxxxx>
>> 
>> On ARM64, arch_sync_dma_for_{cpu,device}() assumes that the
>> physical address passed in refers to normal RAM that is part of the
>> kernel linear(direct) mapping, as it unconditionally derives a CPU
>> virtual address via phys_to_virt().
>> 
>> With Xen swiotlb, devices may use per-device coherent DMA memory,
>> such as reserved-memory regions described by 'shared-dma-pool',
>> which are assigned to dev->dma_mem. These regions may be marked
>> no-map in DT and therefore are not part of the kernel linear map.
>> In such cases, pfn_valid() still returns true, but phys_to_virt()
>> is not valid and cache maintenance via arch_sync_dma_* will fault.
>> 
>> Prevent this by excluding devices with a private DMA memory pool
>> (dev->dma_mem) from the arch_sync_dma_* fast path, and always
>> fall back to xen_dma_sync_* for those devices to avoid invalid
>> phys_to_virt() conversions for no-map DMA memory while preserving the
>> existing fast path for normal, linear-mapped RAM.
>> 
>> Signed-off-by: Peng Fan <peng.fan@xxxxxxx>
>> ---
>>   drivers/xen/swiotlb-xen.c | 17 +++++++++++++----
>>   1 file changed, 13 insertions(+), 4 deletions(-)
>> 
>> diff --git a/drivers/xen/swiotlb-xen.c b/drivers/xen/swiotlb-xen.c
>> index 
>> 2cbf2b588f5b20cfbf9e83a8339dc22092c9559a..b1445df99d9a8f1d18a83b8c413bada6e5579209
>>  100644
>> --- a/drivers/xen/swiotlb-xen.c
>> +++ b/drivers/xen/swiotlb-xen.c
>> @@ -195,6 +195,11 @@ xen_swiotlb_free_coherent(struct device *dev, size_t 
>> size, void *vaddr,
>>   }
>>   #endif /* CONFIG_X86 */
>> +static inline bool dev_has_private_dma_pool(struct device *dev)
>> +{
>> +    return dev && dev->dma_mem;
>> +}
>> +
>
>I don't think this will compile on x86.

My bad, I only tried the build for ARM64. I use below changes, if it looks
good.

#ifdef CONFIG_ARM64
static inline bool dev_has_private_dma_pool(struct device *dev)
{
        return dev && dev->dma_mem;
}
#else
static inline bool dev_has_private_dma_pool(struct device *dev)
{
        return false;
}
#endif

Thanks,
Peng

>
>
>Juergen